The present invention is concerned with the elimination of mercury and possibly arsenic from hydrocarbons. More particularly, the invention relates to a process for which the compounds containing mercury in the charge to be treated are converted into elementary mercury, the charge is fractioned into cuts which are rich in and deprived of mercury, and the cuts containing mercury are then purified through contact with a mercury collecting mass.
It is known that liquid condensates (by-products of gas production) and some crude petroleums can contain a number of metal compounds in trace form and often in the form of organometallic complexes. These metal compounds are very often poisons for catalysts used during the transformation of these cuts into commercial products. Mercury is particularly toxic for the activity of precious metals, and moreover is a corrosive force with aluminium pieces, joints and solders.
Therefore it is advantageous to purify the charges intended to be used in condensate or crude oil transformation processes in order to prevent mercury, and possibly arsenic, from being entrained therein. Purification of the charge upstream of the treatment processes makes it possible for the entire installation to be protected.
The processes heretofore proposed by the Applicant show good demercurisation and de-arsenification performance with the liquid hydrocarbons used as charges in the various treatment processes. U.S. Pat. No. 4,911,825 belonging to the Applicant clearly shows the advantage of collecting mercury and possibly arsenic in a two stage process. The first stage consists in contacting the charge in the presence of hydrogen with a catalyst which contains at least one metal from the group formed by nickel, cobalt, iron and palladium. Mercury is not collected (or not much of it is collected) by the catalyst, but it is activated on the catalyst in such a way as to be collected in the second stage by a mass containing sulphur or sulphur compounds. Patent application WO-90/10684 belonging to the Applicant describes a process for the elimination of mercury and possibly arsenic from liquid hydrocarbons. This invention is concerned with catalysts which have the capacity to withstand sulphur poisoning (thio-resistance). These new catalysts make it possible for mercury and arsenic to be collected when conditions are too severe for the catalysts described in the prior art. They not only contain at least one metal from the group formed by Ni, Co, Fe, Pd but also at least one metal selected from the group formed by chromium, molybdenum, tungsten and uranium.
The process described in this patent is particularly useful for purifying difficult charges such as gas oils originating from fractionation of a crude petroleum wherein the sulphur content is often between 0.4 and 1.0% by weight. On the other hand, the process described in U.S. Pat. No. 4,911,825 performs better with charges with a lower sulphur content, less than 0.15% by weight, for example.